Previously various processes have been developed for treating acid waste liquid for use for metallic materials and articles, for example an acid pickling waste solution of stainless steel, as disclosed in Japanese Patent Application Laid-open Specification Sho No. 48-83097. According to this process, a univalent inorganic acid is removed from an aqueous solution which contains metallic salts of the said univalent inorganic acid. For that purpose, the sulfuric acid content of the solution is adjusted so that the content is at least equivalent to that of the metals, then the solution is brought into contact with an organic solution of a compound which is capable of forming an adduct with the said inorganic acid, to extract the inorganic acid in the organic phase. More particularly, in treating an acid waste liquid which has been used for acid pickling of stainless steels or acid-resistant steels, that amount of sulfuric acid equivalent to the metal contained in the solution is added, then extraction is conducted using an organic phase consisting of 25% kerosene and 75% tributyl phosphate, and finally the acid in the organic phase is stripped with water and returned to the acid washing bath. Remaining acid and molybdenum and other metals are in the outgoing organic phase from the stripping stage washed with a solution of sodium hydroxide. The organic phase is circulated to and used in the extraction stage.
However, the previous process mentioned above has several disadvantages; low recovery of acid, for example hydrofluric acid, which is combined with Fe, low distribution ratio of HNO.sub.3 and HF, large scale installation, increased amount of sludge due to formation of CaF.sub.2 and CaSO.sub.4.2H.sub.2 O, if the solution is neutralized with Ca(OH).sub.2 after the acid has been recovered, high recovering cost of valuable metals like Ni, Cr and Mo by smelting owing to low quality grade of them in the sludge, and coextraction of FeCl.sub.3 as shown in FIGS. 14 and 15 with HNO.sub.3 and HF when hydrochloric acid is used in place of sulfuric acid in order to adjust the hydrogen ion concentration. cl SUMMARY OF THE INVENTION
The present invention has overcome the difficulties of the conventional process as mentioned above, and provides a process for treating acid waste liquids which have been used for acid pickling of metallic materials or articles and for fractionally recovering the mineral acid. Further this invention aims to effectively regenerate the organic solvents which have been used for treating acid waste liquids and also to effectively recover the Fe which has been extracted during the treatment.
More particularly, according to the present invention, an organic solvent (A) is added in the 1st stage to extract and recover Fe in the acid waste liquid into an organic phase; hydrochloric acid is added to the resulting waste liquid not including Fe ions in the 2nd stage to yield extractable undissociated acids (HF and HNO.sub.3) by replacing some of the NO.sub.3.sup.- and F.sup.- in the remaining metal salts with Cl.sup.- and increase the hydrogen ion concentration. Then an organic solvent (B) is added to extract and recover desired mineral acids; and in the 3rd stage an organic solvent (C) is added to the waste liquid which has been treated in the 2nd stage, to extract and recover other desired mineral acids; while organic solvents (A), (B) and (C) are regenerated in the respective stages.
After Fe ions present abundantly in the acid waste liquid are converted into Fe.sup.3+, the organic solvent (A) described above is applied to extract the Fe ions into the organic phase and to recover acid in the waste liquid. The extracted Fe ions in the organic phase are weakly reduced by contacting with a solution containing reducing agents, such as Na.sub.2 SO.sub.4, Na.sub.2 So.sub.4, NaCl NaNo.sub.2 and N.sub.2 H.sub.4, so as to make back-extraction possible. The Fe in the back-extraction solution is recovered by electrolysis in the form of metallic iron. Thus Fe ions and acids are recovered from the acid waste liquid containing Fe ions.